1. Field of the Invention
The present invention generally relates to a thermally-sensible overcurrent protective relay, and more particularly, to an overcurrent protective relay including a wire connecting terminal.
2. Description of the Related Art
Thermally-sensible overcurrent protective relays have been widely used to prevent the overcurrent from being flown through a main device, e.g., induction motors during overload condition. These overcurrent protective relays are known in the field from, for instance, U.S. Pat. Nos. 4,635,020 and 4,652,847 issued to the Applicant.
One of the conventional thermally-sensible overcurrent protective relays will now be described with reference to FIGS. 1 through 7.
FIG. 1 is a front view with a cover 2 removed; FIG. 2 is a sectional view taken along a line A--A in FIG. 1; FIG. 3 is a sectional view taken along a line B--B in FIG. 1; FIG. 4 is a sectional view taken along a line C--C in FIG. 1; FIG. 5 shows a movable contact element; FIG. 6 shows an actuating lever; and FIG. 7 is a perspective view illustrating basic component elements of a snapping inverter.
In FIG. 1, there are shown a case 1, a cover 2, bimetals 3 provided for individual phases (three phases in this example), and heaters 4 wound around the bimetals 3 respectively to generate heat when a main circuit current flows therein. When heated by the heater 4, the bimetal 3 is deformed with a curvature as represented by a dotted line in FIG. 1. A load-side main circuit terminal 5 (FIG. 4) has a tongue 5A to which an upper end of the bimetal 3 is joined and secured. The load-side main circuit terminal 5 is anchored to the case 1 by means of a clamp screw 6, and a terminal screw 7 for connecting a load-side main circuit (external circuit) is fastened to one end 5B of the terminal 5. Also, a lower end 4B of the heater 4 is electrically connected to a lower end of the bimetal 3 by some suitable means such as welding.
In a main circuit terminal for a power supply side 40, as shown in FIG. 4, an upper end 4A of the heater is electrically connected to its one end 40A by welding or similar means. Meanwhile, a left end 40B of the main circuit terminal 40 is screwed to a terminal of a power supply circuit used for an electromagnetic contactor (not shown) and so forth. A communicating plate 8 is kept in engagement with the fore end of the bimetal 3 of each phase so as to transmit the deformation of the bimetal 3. In the example of FIG. 1, the communicating plate 8 is so disposed that its left end depresses a lower end of a temperature compensating bimetal 9. Further, an actuating lever 10 is disposed to be rotatable around a shaft 11 with an upper end of such temperature compensating bimetal 9 anchored to the lever 10 (see FIG. 1).
The shaft 11 is held at its two ends by a lever supporting member 12 as shown in FIG. 3. The lever supporting member 12 is retained, at an inner corner 12A of its L-shaped bend, in abutment against an edge 1A of the case 1 and is thereby held at a fulcrum while being pressed against an adjusting screw 13 through a first tongue 12B. In the meanwhile, a second tongue 12C is elastically urged leftward, as viewed in FIG. 1, by a leaf spring 14.
Consequently, the lever supporting member 12 is rotatable around the edge 1A by turning a control knob 15 disposed above the adjusting screw 13. In addition, the shaft 11 attached to the lever supporting member 12 is positionally changed substantially in the horizontal direction in FIG. 1, thereby controlling the operating current in response to the curvature of the bimetal 3 curved by the current generated from the heater 4.
A movable contact element 16 is composed of a thin metal plate having sufficient elasticity and conductivity. As illustrated in FIG. 5, the movable contact element 16 is produced by punching a plate to have an inner beam portion 16A and outer beam portions 16B. A U-shaped leaf spring 17 is interposed between the fore end of the inner beam portion 16A and the outer beam portions 16B in such a manner as to depress the contact element 16 with elastic urge. A contact portion 16C of the movable contact element 16 is disposed opposite to and in abutment against a fixed contact element 18 for a normally closed contact, thereby constituting a normally closed contact mechanism. Then a lower end 16E of the movable contact element 16 shown in FIG. 5 is clinched firmly via a through hole 16G to a normally closed movable terminal 19 shown in FIG. 1. This terminal 19 is anchored to the case 1 by means of a clamp screw 20 as illustrated in FIG. 3.
The inner beam portion 16A of the movable contact element 16 is inserted into a substantially T-shaped slit 10A formed at the fore end, or tip of the actuating lever 10 shown in FIG. 6. An upper end 16F extending from the outer beam portion 16B of the movable contact element 16 is engaged with a groove 21A formed at the left end of a cross bar 21. The cross bar 21 is guided by the case 1 to be movable horizontally, as viewed in FIG. 1.
Each of a normally-open fixed contact element 24 and a normally-open movable contact element 25 is composed of a thin metal plate having sufficient elasticity and conductivity. Such two contact elements 24 and 25 are clinched and fastened respectively to a normally open fixed terminal 22 and a normally-open movable terminal 23 shown in FIG. 2. A back surface 25A of the upper distal end of the normally-open movable contact element 25 in its positional change is disposed in abutment against a projection 21G of the cross bar 21. A reset bar 26 is held slidably by the case 1 and is displaceable vertically in FIG. 1. Normally the reset bar 26 is elastically urged at its edge 26C upward by a return spring 27 and is retained at an upper-limit halt point. In this state, a lower vertical plane 26D of the reset bar 26 is kept in abutment against a curved portion 24A formed on a back surface of the normally open fixed contact element 24. Then, an inclined portion 26A of the reset bar 26 is slid and depressed against such curved portion 24A in accordance with the downward displacement of the reset bar 26, thereby displacing the normally-open fixed contact element 24 rightward in FIG. 1.
When such conventional thermally-sensible overcurrent protective relay is used in an auto-reset system, first the reset bar 26 is depressed downward to displace a changeover plate 30 leftward in FIG. 1, so that the fore end of the changeover plate 30 is inserted into a lock hole 26B formed in the reset bar 26, and the protrusion lB of the case 1 is fitted into a recess on the bottom of the changeover plate 30, whereby the reset bar 26 is restricted with respect to its upward return.
In the conventional thermally-sensible overcurrent protective relay of the structure mentioned, the following operation is performed.
In FIG. 4, a main circuit current flows from the main circuit terminal for the power supply side 40 via the heater 4 and the bimetal 3 to the load side main circuit terminal 5. An electric wire (not shown) is connected to the terminal screw 7 fastened to one end 5B of the load-side main circuit terminal 5 and is further connected to a load (not shown) such as an induction motor. Consequently, the main circuit current becomes equivalent to the load current. Due to the Joule heat loss caused by such main circuit current in the bimetal 3 and the heater 4, the bimetal 3 is heated and curved as represented by a dotted line in FIG. 1.
Upon occurrence of an overcurrent condition in the load, the main circuit current becomes higher to further increase the curvature (bending curve) of the bimetal 3 represented by the dotted line in FIG. 1, hence causing its further displacement leftward. As a result, the communicating plate 8 is depressed by the fore end of the bimetal 3 and is thereby displaced leftward in FIG. 1. In response to such leftward displacement of the communicating plate 8, a coupled assembly of the temperature compensating bimetal 9 and the actuating lever 10 is pressed by the left end of the communicating plate 8 and is thereby rotated clockwise around the shaft 11, so that the inner beam portion 16A of the movable contact element 16 in abutment against the periphery of the substantially T-shaped slit 10A at the fore end of the actuating lever 10 is bent rightward in FIG. 1.
When the inner beam portion 16A thus bent and displaced has reached a dead center point determined by the relationship between the elastic urge of the U-shaped leaf spring 17 and the spring force of the outer beam portion 16B of the movable contact element 16 for returning to the former state, the movable contact element 16 is suddenly inverted to induce leftward jump of the outer beam portion 16B and rightward jump of the inner beam portion 16A in FIG. 1.
Therefore, the normally-closed contacts held in electric conduction are opened by the abutment of the contact portion 16C against the fixed contact element 18 for the normally-closed contact, hence interrupting the main circuit.
Meanwhile, the cross bar 21 is pulled by an upper end 16F of the outer beam portion 16B and is thereby shifted leftward in FIG. 1, so that the projection 21G serves to displace the normally-open movable contact element 25 leftward. Consequently, the normally-open movable contact element 25 is brought into abutment against the normally-open fixed contact element 24 to eventually cause electric conduction.
Therefore, by connecting the normally-closed contact in series with the operating coil circuit (not shown in detail) of an electromagnetic contactor (not shown) which switches on and off the main circuit, it is rendered possible to interrupt and protect the main circuit upon occurrence of an overcurrent condition in the load (not shown) such as an induction motor. Furthermore, an overload alarm signal may be produced by connecting an alarm lamp or equivalent circuit in series with the normally-open contact.
After generation of thermal energy from the heater 4 is ceased as a result of interruption of the main circuit current and the bimetal 3 is cooled to resume the former state, both the normally-open and normally-closed contacts can be returned to the former positions thereof by external manual actuation to depress the reset bar 26 downward in FIG. 1. When the reset bar 26 is manually depressed downward in FIG. 1 against the elasticity of the return spring 27, the inciined portion 26A of the reset bar 26 presses rightward the curved back portion 24A of the normally open fixed contact element 24, which is thereby bent rightward in FIG. 1. Consequently, the normally movable contact element 25 held in abutment against the normally-open fixed contact element 24 is displaced rightward, so that the cross bar 21 is also displaced rightward in FIG. 1 with its projection 21G being pressed by the back surface 25A of the normally open movable contact element 25.
In the conventional thermally-sensible overcurrent protective relay as mentioned above, the following problems will be considered in the case of tightening the wire connecting terminal screw 90 of the terminal 19 for the normally-closed movable contact during the wiring connection (see FIG. 3).
The terminal 19 for the normally-closed movable contact is fixed by the tightening screw 20 so as to prevent the change in position due to the tightening operation of the terminal screw 90 shown in FIG. 3. However, the tightening torque of the terminal screw 90 exceeds that of the tightening screw 20, and there sometimes occurs a slight change in position (a change in rotational position about the axis of the tightening screw 20 as shown in FIG. 3). Since the movable contact 16 is electrically and mechanically connected with the terminal 19 for the normally-closed movable contact at a free end 16E (see FIG. 7) of the contact 16 by means of clinching or the like, the aforementioned slight change in rotational position is enlarged at the end (tongue 16F) of the movable contact 16, causing the shift of a reverse position. In other words, the shift of the reverse position causes the shift of the operational position of the bimetal 3, resulting in deterioration in a protective function against the overcurrent.
Accordingly, the present invention has been accomplished in an attempt to overcome the above conventional problems, and it is therefore an object of the present invention to provide a thermally-sensible overcurrent protective relay which achieves no shift in the protective operational point against the overcurrent even when the wire connecting terminal screw is tightened for the purpose of the connection to an external wiring.